The gut epithelia of most metazoan organisms encompass complex microbial communities that range from autochthonous bacteria to allochthonous bacteria. Autochthonous bacteria have evolutionarily adapted to the host gut environment and are capable of colonizing the gut by permanent attachment thereto, whereas allochthonous bacteria are introduced from external environments and transiently interact with gut epithelia by passing through the alimentary flowing stream. Therefore, in response to various and continuous influx of microbes, host microbes have evolved to modulate the gut immunity to achieve gut-microbe homeostasis.
As such, all metazoan guts possess immunologically unique environments. Specifically, these environments enable the commensal microbes among various microbes derived from external environments to have a symbiosis while having an efficient antimicrobial system for eliminating pathogens. However, this is a paradoxical situation from the classical view point of innate immunity because host immune cells should be able to mount an antimicrobial response against any microbes, regardless of whether it is commensal or pathogenic, by sensing universal microbe-associated molecular patterns (MAMPs).
Several models have been proposed to explain this paradoxical situation. These models include restricted expression and compartmentalization of pattern recognition receptors (PRRs), multiple mechanisms to down-regulate NF-kappaB-dependent innate immune signal pathways, and compartmentalization of gut bacteria by the mucus layer (Hooper et al., Nat Rev Microbiol 7, pp. 367-374, 2009; Lhocine et al., Cell Host Microbe 4, pp. 147-158, 2008; Paredes et al , Immunity 35, pp. 770-779, 2011; Ryu et al., Science 319, pp. 777-782, 2008). However, the molecular mechanism determining how the gut tolerates symbiotic bacteria without mounting inflammation remains to be elucidated. If the molecular mechanism is understood, it would enable beneficially established commensal microbes to live in a symbiotic relationship and secure a target for controlling the gut immune environment that stimulates elimination of pathogens.
As such, the present inventors have disclosed a direct role of DUOX in gut immune response in Drosophila in previous papers (Ha et al., Science, Vol. 310, pp. 847-850, 2005). It has been confirmed that DUOX, which is an NADPH oxidase existing in the mucous membrane of Drosophila, plays a pivotal role in immunological activity for microbes by utilizing a genetically recombined Drosophila model. Further, as the mechanism for controlling DUOX, the present inventors have disclosed that DUOX activation involves Gαq-PLCβ-Ca2+-mediated signaling pathways and that DUOX expression modulates DUOX gene induction through sequential activation of MEKK1-MKK3-p38 MAPK (Ha et al., Dev Cell 16, pp. 386-397, 2009; Ha et al., Nat Immunol 10, pp. 949-957, 2009b).
In addition to DUOX-dependent gut immunity, pathogen infection can also activate the immune deficiency (IMD) pathway and subsequent nuclear localization of the Relish and NF-kappaB protein, which in turn leads to de novo production of AMPs. DUOX-dependent ROS and IMD-dependent AMP act synergistically or mutually in the gut. The IMD activation is known to be stimulated by peptidoglycan on the cell surface. However, DUOX activity was not activated by peptidoglycan. Instead, the protein-derived ligand for DUOX activation activates DUOX through G-protein-coupled receptor (GPCR) (Bae et al., Trends Immunol 31, pp. 278-287, 2010; Ha et al., Nat Immunol 10, pp. 949-957, 2009b). Despite the presence of extremely efficient DUOX-dependent antimicrobial activation against pathogenic allochthonous bacteria, symbiotic autochthonous bacteria can still colonize the gut without DUOX activation and play their part in maintaining gut-microbe mutualism. However, the microbe-derived factors involved in the activation of DUOX-mediated gut immunity have not yet been determined.
Therefore, the present inventors confirmed that DUOX stimulates a secretion of oxidants in gut cells upon microbial infection, thereby directly acting on gut immunity. However, the reason why gut immunity caused by DUOX is not activated by commensal microbes or the molecular mechanism causing immunological activity is yet to be determined.
On the other hand, uracil, which is one of the nucleobases forming a pyrimidine series of RNA, forms mRNA by combining with adenine in a complementary manner during the transcription process. Uracil is not specifically for pharmaceutical use. However, 5′-fluorouracil (5′-FU), which is an analog of uracil and acts as an antagonistic agent to the pyrimidine nucleobase, is used as a major antitumor agent through antagonistic hexane metabolism, such as DNA synthesis inhibition or RNA dysfunction. Fluorouracil, the uracil analog, is known to cause defects in some mucous membranes of the digestive tract, but the defects are only known to be failures caused by the prevention of cell division and cell denaturation.